How do I iterate over a pointer to a pointer?

问题:

Lets say I have the following variables:

char c[] = "ABC";
char *ptr = &c;
char **ptr2 = &ptr;

I know I can iterate over a pointer to an array of char, this way:

int i;
for(i=0; i<3; i++){
    printf("TEST******************, %c\n", ptr[i]);
}

How do I iterate over a pointer to a pointer?

回答1:

For your example:

int i;
for(i=0; i<3; i++){
    printf("TEST******************, %c\n", (*ptr2)[i]);
}

回答2:

Suppose:

  6   char c[] = "ABC";
  7 
  8   char *ptr   = &c;
  9   char *ptr2  = ptr;       
 10   char **ptr3 = &ptr;   

In this scenario:

• ptr represents an address of c
• ptr2 represents an address of ptr. A pointer to a pointer
• ptr3 is a value stored in ptr, which is an address of c.

**ptr3=&ptr means - Take address of ptr, look inside and assign its value (not address) to ptr3

If I understood your question correctly, you need to use pointers to pointers: ptr2 in my example instead of ptr3

If so, you can access elements like :

ptr2[0] = A
ptr2[1] = B
ptr2[2] = C

For the record the following will yeld the same results. Try it.

 12   printf ("===>>> %x\n", ptr2);
 13   printf ("===>>> %x\n", *ptr3);

Good discussion for your reference is here

回答3:

If I did not misunderstand your question, this code should make a job

printf("TEST******************, %c\n", (*ptr2)[i]);

回答4:

let me give an example,

char **str; // double pointer declaration
str = (char **)malloc(sizeof(char *)*2);
str[0]=(char *)"abcdefgh";   // or *str  is also fine instead of str[0]
str[1]=(char *)"lmnopqrs";

while(*str!=NULL)
{
    cout<<*str<<endl; // prints the string
    str++;
}
free(str[0]);
free(str[1]);
free(str);

or one more best example which you can understand. here i have used 2 for loops because i am iterating over each character of the array of strings.

char **str = (char **)malloc(sizeof(char *)*3); // it allocates 8*3=24 Bytes
str[0]=(char *)"hello";    // 5 bytes 
str[1]=(char *)"world";    // 5 bytes
// totally 10 bytes used out of 24 bytes allocated
while(*str!=NULL)   // this while loop is for iterating over strings
{
    while(**str!=NULL)   // this loop is for iterating characters of each string
    {
         cout<<**str;
    }
    cout<<endl;
    str++;
}
free(str[0]);
free(str[1]);
free(str);